The invention resides in an internal combustion engine with an exhaust gas turbocharger including an exhaust gas turbine arranged in the exhaust system and a charge air compressor arranged in the fresh air supply of the engine and driven by the exhaust gas turbine.
For the recirculation of exhaust gases of supercharged internal combustion engines for limiting nitrogen oxide emissions, recirculation systems are generally employed in today's engines which extract the exhaust gas to be recirculated from the exhaust system between the engine discharge and the exhaust gas turbine. The exhaust gas is then conducted possibly through a separate heat exchanger and returned to the fresh intake air duct generally downstream of a charge air cooler. Because of the characteristics of the exhaust gas turbocharger, the useable range for exhaust gas recirculation ends where the charge air pressure downstream of the charge air compressor exceeds the exhaust gas pressure upstream of the exhaust gas turbine since then the pressure drop required for the exhaust gas recirculation is no longer available.
DE 42 31 218 C1 discloses an exhaust gas recirculation system wherein the exhaust gas is extracted from the exhaust system in the manner as just described upstream of the exhaust gas turbine and is then pressurized by a separate exhaust gas compressor to a higher pressure so that it can be subsequently admixed to the fresh charge air downstream of the air compressor. The amount of the recirculated exhaust gas is controlled by way of a 3/2 way valve. The separate exhaust gas compressor is driven by the exhaust gas turbine of the exhaust gas turbocharger. In another embodiment, the exhaust gas is compressed by second exhaust gas turbocharger arranged in series with the first exhaust gas turbocharger.
It is however a disadvantage of this exhaust gas recirculation system that it negatively affects the charge change or the engine since exhaust gas is removed upstream of the exhaust gas turbine of the exhaust gas turbocharger whereby the turbine power drops with increased exhaust gas recirculation and accordingly the desirable charge air pressure cannot be reached. This leads to high particle emissions of the engine and to a limitation of the exhaust gas recirculation range since, during exhaust gas recirculation, particularly during full or almost full-load operation of the engine, the combustion air ratio during exhaust gas recirculation is already very critical even with an exhaust gas turbine not disturbed by exhaust gas recirculation. This form of exhaust gas recirculation is also critical with regard to the nonstationary behavior of the internal combustion engine. With ram charging which is common in internal combustion engines, the very high additional volumes of the exhaust gas turbine of the turbocharger detrimentally affect the charging process. In addition, the compressor wheel for the compression of the exhaust gas increases the mass inertia moment of the exhaust gas turbocharger and consequently detrimentally affects the acceleration behavior.
Furthermore, during stationary exhaust gas recirculation operation, the efficiency will probably suffer since the charge air compressor for the fresh air then operates outside its full load design operating range and therefore at relatively low efficiency because only the amount of fresh air without the exhaust gas flow must be compressed, whereas the charge air compressor must be designed such that it can supply the full amount of air required by the internal combustion engine when the exhaust gas recirculation is not in operation.
In an exhaust gas recirculation system with a serial exhaust gas turbocharger, it is disadvantageous that the turbine is always rotating even when no exhaust gas is needed. This detrimentally affects the gas change of the internal combustion engine because of the higher efforts required for discharging the exhaust gas by the piston. With both embodiments, it is uneconomical from the point of view of thermodynamics to first withdraw the operating medium so that it is not supplied to the turbine then compress it and then release excess exhaust gas without using it.
EP 0 620 365 A1 discloses an internal combustion engine with exhaust gas recirculation wherein a conventional exhaust gas turbocharger is provided and a second exhaust gas turbocharger is arranged parallel to the conventional exhaust gas turbocharger solely for compressing exhaust gas. In this case, the exhaust gas to be recirculated is extracted only downstream of the turbine, but the partial exhaust gas flow which serves for driving the second exhaust gas turbocharger is not available for driving the conventional exhaust gas turbocharger. In such an arrangement, the non-stationary behavior is even less satisfactory since during acceleration, it is necessary to shut down the exhaust gas recirculation in order to prevent inadmissible particle emissions whereby first the whole second exhaust gas turbocharger must be uncoupled. Only in this way can the exhaust gas flow through the main (conventional) turbine and the speed of the exhaust gas turbocharger be increased. The partial exhaust gas flow for driving the second turbocharger for the exhaust gas recirculation is therefore lost for the main turbine. For vehicle drive arrangements and ram charging wherein the pressure peaks in the exhaust gas system are utilized such an arrangement is really unsuitable because of the large detrimental volumes in the exhaust gas system. Just like in the previously described state of the art, it is also here disadvantageous that the compressor and the turbine of the main exhaust gas turbocharger need to cover too large a range so that they cannot be tuned for optimal efficiency. In addition, this system is expensive to build, has large space requirements and also the equipment costs are relatively high.
DE 40 07 516 A1 discloses a Diesel engine wherein exhaust gas and air are compressed together in a single compressor. Such an arrangement however requires a particle filter to prevent deposits in and wear of the compressor and any intercooler arranged downstream of the compressor. The expenses for a particle filter and the regeneration equipment therefor are a disadvantage in such an arrangement.
It is therefore the object of the present invention to provide an internal combustion engine of this type in which however, the effect of the exhaust gas recirculation is available for the whole engine operating range without detrimental effects on the charge change of the engine and on the non-stationary behavior of the engine and wherein relatively few building components are required for the exhaust gas recirculation.